1. Field of the Invention
The present invention relates to an ultrasonic diagnosing apparatus for obtaining blood flow data as function information associated with the movement of tissues due to the function of an organ in a living body by utilizing an ultrasonic Doppler method, and for displaying the data as an image.
2. Description of the Related Art
A certain type of ultrasonic diagnosing apparatus is designed to obtain blood flow data by utilizing the ultrasonic Doppler method as well as a B-mode image (tomographic image) and an M-mode image, which are obtained by transmitting/receiving an ultrasonic wave to/from a living body as an object to be examined. In such an apparatus, measurement of a blood flow velocity based on the ultrasonic Doppler method is performed as follows. When an ultrasonic wave is transmitted to a blood flow in a living body, the ultrasonic wave is scattered by flowing blood cells. The ultrasonic echo obtained as the reflected ultrasonic wave is subjected to a Doppler shift due to the movement of blood cells upon reflection, and its frequency is changed. That is, if the center frequency of the transmitted ultrasonic wave is fc, the frequency of the ultrasonic echo subjected to a Doppler shift is changed by fd, and a frequency f of the received ultrasonic echo is given by: EQU f=fc+fd
In this case, the frequencies fc and fd are represented by the following equation: EQU fd=2vcos.theta..multidot.fc/C
where
v: blood flow velocity PA1 .theta.: angle defined by ultrasonic beam and blood vessel PA1 C: velocity of sound
Therefore, the blood flow velocity v can be obtained by detecting the Doppler shift fd.
Two-dimensional image display of the blood flow velocity obtained in this manner is performed in the following manner.
Assume, as shown in FIG. 1, that scanning control of an ultrasonic wave based on so-called sector scan is performed by sequentially transmitting an ultrasonic beam as a pulse from an ultrasonic transducer 1 in respective directions D1, D2, D3, ... Dm. In sector scan by the ultrasonic transducer 1, in many cases, an array transducer in which a plurality of transducer elements are arranged is used, and a so-called sector electronic scan method is employed. In this method, transmission/reception of an ultrasonic wave by the plurality of transducer elements is repeated while the driving timings and/or processing timings of received signals of the respective transducer elements are sequentially and electronically shifted from each other, thus sequentially changing the steering angle of the ultrasonic beam to be transmitted/received. An ultrasonic transducer 1 and a linear scan method as a scanning control method thereof may also be employed. In this case, generally, a predetermined number of transducer elements of a plurality of ultrasonic transducer elements constituting an array transducer are used as a group. One transmission/reception operation of an ultrasonic beam is performed by using the group of transducer elements. By sequentially switching the transducer elements selected as the elements of the group, the transmission/reception position of the ultrasonic beam is horizontally moved. For example, the transducer elements of the group are sequentially shifted and selected one by one so as to electronically shift the transmission/reception position of the ultrasonic beam. In these scan operations, the excitation timings and/or reception signal timings of selected ultrasonic transducer elements located at central portion and a peripheral portion of the beam are shifted from each other so that the ultrasonic beam can be substantially focused by utilizing a difference in phase between sound waves generated by the respective transducer elements or reception signals thereof. This operation is called electronic focusing.
When blood flow data is to be obtained by the Doppler method, ultrasonic pulses are transmitted several times in a direction, e.g., the direction D1 in FIG. 1. The transducer 1 is driven by a transmitter/receiver 2 shown in FIG. 2 so as to transmit an ultrasonic wave into an object to be examined. The ultrasonic wave is then reflected by a blood flow (blood cells) in the object, and the reflected wave, i.e., the ultrasonic echo is received by the same transducer 1. This reflected ultrasonic wave is converted into an electrical signal by the transducer 1 and the transmitter/receiver 2.
The electrical signal is then phase-detected by a phase detector 3 so as to detect a Doppler shift signal. This Doppler shift signal is sampled at each of 256 sampling points SP set in the beam direction of the ultrasonic pulse. The Doppler shift signal at each sampling point SP is A/D (analog-to-digital)-converted by an MTI (moving target indicator) circuit 4a, and is subjected to MTI processing including frequency analysis. The obtained Doppler data is output to a DSC (digital scan converter) 6. The Doppler data is converted according to a scan system for display by the DSC 6 and is output to a display section 7.
In this manner, a blood flow velocity distribution in the direction D1 in FIG. 1 can be obtained as one-dimensional data in a real-time manner. A similar operation is repeated in the directions D2, D3, ... Dm, and one-dimensional data of a blood flow velocity distribution in each beam direction can be obtained. As a result, a two-dimensional blood flow image (blood flow velocity distribution image) is displayed on the display section 7.
The blood flow detection resolution (detection performance) at low velocity depends on the time length of data to be frequency-analyzed. If the sampling frequency of a Doppler signal is represented by fr, and the number of sampled data at one point is represented by n, a time length T of data to be frequency-analyzed is given by: EQU T=n/fr (1)
In this case, a frequency resolution fd is given by: EQU fd=1/T (2)
Therefore, a lower limit fdmin of a measurable flow velocity is represented as follows: EQU fdmim=1/T=fr/n (3)
In order to detect a blood flow at low velocity, therefore, either the sampling frequency fr of a Doppler signal is reduced, or the number n of data is increased.
In two-dimensional Doppler imaging, however, EQU FN.multidot.n.multidot.m.multidot.(1/fr')=1 (4)
(where FN: frame frequency; m: number of ultrasonic beam scanning lines; fr'; pulse repetition frequency or pulse rate frequency) The frame frequency FN is associated with the real time properties of a two-dimensional blood flow image and is normally 8 to 30. At this frequency, 8 to 30 frames can be obtained per second.
In the sector electronic scan method, if the number m of ultrasonic beam scanning lines (the number of scanning lines in the beam direction in ultrasonic scan) =32, the pulse repetition frequency fr'=4 kHz, and the number n of sampled data=8, the frame frequency FN is about 16.
A maximum depth of view field Dmax and the pulse repetition frequency fr' have the following relationship: EQU Dmax=C/(2.multidot.fr') (5)
If, therefore, the pulse repetition frequency fr' is increased in order to improve the detection resolution at low velocity, the maximum depth of view field cannot be increased. In addition, if the number m of ultrasonic beam scanning lines is decreased, the ultrasonic beam scanning line density is reduced, resulting in degradation of image quality.
If the detection resolution at low velocity is improved in this manner, other characteristics are degraded.
Under the circumstances, for example, Published Unexamined Japanese Patent Application No. 64-43237 (U.S. patent application No. 228,590) discloses an apparatus employing an alternate scan method as a method of solving such a problem.
In this method, the scanning order of an ultrasonic beam is changed, as shown in FIGS. 3A and 3B.
Referring to FIG. 3A, when an ultrasonic transmission beam is to be scanned from the right end of a sector region to be scanned by an ultrasonic transducer 1, the scanning order is first the first right beam scanning line (No. 1), then the second beam scanning line (No. 2), the third beam scanning line (No. 3), the fourth beam scanning line (No. 4), the first beam scanning line (No. 1), the second beam scanning line (No. 2), the third beam scanning line (No. 3), .... That is, block scan is performed every four beam scanning lines. In this case, the repetition frequency fr of an ultrasonic transmission beam in the same direction is given by: EQU fr=fr'/4 (6)
As is apparent from equation (3), if the method shown in FIG. 3A is employed, the lower limit fdmin of measurable flow velocity can be reduced to 1/4 that of the conventional method, i.e., transmitting an ultrasonic pulse n times in a first beam direction, and sequentially transmitting an ultrasonic pulse n times in the adjacent beam directions.
In this case, according to the method shown in FIG. 3A, if the number of ultrasonic transmission operations in the same beam direction (the number of sampling operations of Doppler signals) is represented by n, n =8. If, however, eight data are read out from a memory (not shown) for each beam scanning line, since block scan is performed every four beam scanning lines, as shown in FIG. 3B, the time phase differences among the respective blocks are large, resulting in a discontinuous image within one frame.
As a method of reducing the above-mentioned time phase difference, the above-described patent application also discloses an alternate constant-interval scan method as shown in FIGS. 4A to 4C. In this method, a plurality of ultrasonic beam scanning lines are sequentially scanned as a group, and group scanning is performed by sequentially changing the ultrasonic beam scanning lines selected as the group while performing sequential scanning within the group, thus performing beam scanning of the entire scanning region. The scanning direction of the ultrasonic beam scanning lines in each group is the same as the scanning direction of group scanning performed by sequential changes in selection of groups. When scanning is to be performed from the right end of a transducer 1 in FIG. 4A, the scanning order is first a beam scanning line No. 1, then a beam scanning line No. 2, a beam scanning line No. 3, a beam scanning line No. 4, a beam scanning line No. 1, then a beam scanning line No. 2, a beam scanning line No. 3, a beam scanning line No. 4, ... (No ultrasonic wave is actually transmitted to "dummy" portions. However, for the sake of easy understanding, a description of the "dummy" portions is omitted, and this scan method will be described after the "dummy" portion), as shown in FIGS. 4A and 4C. Although no ultrasonic wave is actually transmitted to the "dummy" portions in FIG. 4A, scanning is performed at corresponding timings. With this operation, similar to the case shown in FIGS. 3A and 3B, the repetition frequency of an ultrasonic beam (the sampling frequency for Doppler signals) fr in the same beam direction can be reduced to 1/4 that of the conventional apparatus. In addition, since the output timings of Doppler data can be set at equal intervals, the time phase differences in one frame can be set to be uniform.
Even in such a method, however, the following problem is posed. Assume that a significantly reflecting substance by which a ultrasonic wave is significantly reflected is present outside the depth of view field. In this case, as shown in FIGS. 4A, 5A, and 5B, since the ultrasonic beam direction of the immediately preceding scanning is changed at the end of a plurality of sampling operations in the same beam direction, an echo signal from outside the depth of view field may enter the next rate period (scanning period). For this reason the echo signal becomes a residual echo signal and is regarded as a phase difference, thus producing a color artifact Q in the depth of view field. For example, in the case shown in FIGS. 4A to 4C, since the rate immediately before the ultrasonic beam scanning line No. 3 may coincide with that of the beam scanning line No. 2, No. 6, or No. 7, a phase difference is generated due to the residual echo. This phase difference becomes a Doppler signal to produce an artifact, resulting in poor image quality.